OCLP not allowing 2017 5K iMac to use Target Display Mode

I just got a new M2 MacBook Pro and I want to use my old 2017 5K iMac as an external display. I've seen several reports saying this combination of a 2017 5K iMac and an M* Mac works, but I can't get this to work at all.

  • iMac has Monterey installed with latest OCLP
  • M2 is running Sonoma
  • Command-F2 on iMac does nothing, I even tried using the virtual keyboard. Should Cmd-F2 do something, even if another Mac is connected or not?
  • Both Macs see each other in System Report under Thunderbolt.
  • Both Macs are connected using a Thunderbolt 4 cable.
  • I tried rebooting with or without the Thunderbolt cable connected to the M2.
  • Post-install root match says that no patches are required.
  • I tried this with Ventura also.

I'm at a loss as to what could be wrong. What can I do to debug this, or are there any logs or config files that could clue me in as to what's wrong? Thanks.

submitted by /u/starmike
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